Automated wafer handling equipment is heavily employed during the fabrication of semiconductor wafers. Generally speaking, wafer handling equipment includes those devices used to transfer a wafer from one location to the next or to secure a wafer during a particular process. It is critical that these devices engage the wafer in such a way that refrains from damaging the fragile circuits located on the wafer face. One apparatus that satisfies this requirement is a vacuum-assisted coupling device. This device operatively engages the wafer surface with sufficient vacuum pressure to secure the wafer thereto. Although various vacuum-assisted devices are used throughout wafer fabrication, the present invention relates primarily to a vacuum-assisted retaining chuck for securing a wafer during processing and the remainder of the discussion will focus on the same.
The vacuum chuck has a generally planar engaging surface for coupling to the wafer surface. In communication with the engaging surface is a vacuum port. The chuck is brought into contact with a first or face side of the wafer and a vacuum source coupled to the vacuum port is activated. As the vacuum is drawn, the wafer is pulled into contact with the engaging surface. The combination of vacuum pressure and the frictional forces resulting between the wafer and the engaging surface securely retain the wafer relative to the chuck. Vacuum chucks are used on many wafer processing machines including, for example, backgrinders and wafer inspection equipment. Unlike vacuum transfer devices which need only support the wafer during transport, the vacuum chuck rigidly holds the wafer while the latter is subjected to various external forces introduced during wafer processing.
While the vacuum chuck performs more than adequately with flat wafers, problems occur when the wafer is warped. Wafer warpage may result from various thermal, mechanical, and chemical processes. Since a warped wafer cannot lie flat against the engaging surface of the vacuum chuck, vacuum leaks form therebetween. These leaks prevent the chuck from adequately sealing with the wafer. As a result, the wafer may shift or even dislodge from the chuck.
To alleviate this problem, some chucks provide an O-ring seal member proximal the perimeter of the chuck. The O-ring acts as a cushion and can improve sealing against wafers having slight warp. Unfortunately, the O-ring is ineffective in attempting to seal against wafers having significant warp.
Current techniques used to couple warped wafers to the vacuum chuck include, for example, utilizing relative high vacuum pressure. Unfortunately, higher vacuum pressure requires more expensive vacuum sources and more energy to operate. Another wafer coupling technique involves pressing the wafer against the chuck, generally by hand, as the vacuum is drawn. By momentarily deflecting the wafer in this way, the planar surface seals against the wafer and permits an effective vacuum. While effective in some circumstances, this practice is time consuming and requires operator intervention. In addition, this manual technique may cause damage to the wafer surface. Furthermore, if the wafer slips off the chuck, potential injury to the operator may result.
Thus, there are unresolved issues with current wafer vacuum chucks. What is needed is a chuck that can easily couple to a significantly warped wafer. What is further needed is a vacuum chuck that can couple to a warped wafer without operator intervention and while operating at relatively low vacuum pressures.